The present invention relates to a molding apparatus for molding an optical element such as a glass-molded lens from a thermally softened glass material by pressing a mold, an optical element molding method, and an optical element and prism as molded products.
Conventionally, to obtain a glass-molded lens, a molding apparatus, the main section of which is shown in FIG. 1, is used. In FIG. 1, a glass gob 1 having a predetermined weight is placed between lower and upper molds 2 and 3 slidably arranged in a cylindrical mold 4, and is softened by heating. The lower and upper molds 2 and 3 are made relatively come close to each other to press-mold the glass gob 1, thereby obtaining a required optical glass element such as a glass-molded lens.
With the molding apparatus having this mold structure, the cavity formed in the cylindrical mold 4 is a closed space partitioned by the molding surfaces of the lower and upper molds 2 and 3. When the glass-molded lens is obtained by this apparatus, variations in weight of the glass gob directly appear as variations in thickness of the lens to adversely affect the optical characteristics of the lens.
In order to eliminate the influence cased by the variations in weight of the glass gob, a countermeasure for this is proposed. As shown in FIG. 2, when press molding is performed with upper and lower molds 7 and 6 so as not to regulate the periphery of the glass material with the inner surface of a cylindrical mold 8, an excess 5 of the glass gob is allowed to swell from the edges of the molding surfaces and is received in a recess formed in the inner surface of the cylindrical mold 8.
When the recess is formed in this manner such that the entire periphery of the side surface of the lens to be molded forms an equal swell-out portion, the swell-out portion must be additionally formed with a portion that serves as a reference when incorporating the molded lens in a predetermined holding frame.
In order to avoid this additional formation, a molding apparatus shown in, e.g., Japanese Patent Laid-Open No. 60-171233 is employed. In this apparatus, as shown in FIGS. 3 and 4, slide cores 10L and 10R that slide laterally are molded at portions that form the side surface of the lens, and are moved forward/backward by actuators 11L and 11R. Spaces 9 into which the excess of the glass gob can flow are formed at required portions of the slide cores 10L and 10R. Hence, the lens can be molded with positional references, in addition to the glass swell-out portion, in the horizontal direction simultaneously with formation of optical functional surfaces.
In order to form the lens surfaces and the lens positioning references simultaneously, Japanese Patent Publication No. 63-10100 has already proposed a molding apparatus necessary for this. As shown in FIG. 5, a side surface regulating portion 17 is formed only on the inner surface of a cylindrical mold 16 which does not positionally regulate swelling from the edge of the molding surface of an upper mold 15 but is in contact with the edge of the molding surface of a lower mold 14 corresponding to the optical functional surface of the lens. Alternatively, as shown in FIG. 8, upper and lower upper position regulating members 24 and 23 that form position regulating portions 28 and 27 are formed on the edges of the molding surfaces of upper and lower molds 22 and 21 in a cylindrical mold 25. The molding apparatus has both a portion that regulates circumferential swelling of the softened glass gob from the edges of the upper and lower molds, and a portion 18 (see FIG. 5) or 26 (see FIG. 8) that does not regulate the softened glass gob but allows it to swell out.
A molded optical element as the target of the proposed molding apparatus is axially symmetrical. The amount of excessive glass swelling out from the edges of the molding surfaces of the upper and lower molds when performing press molding can be automatically set constant highly precisely if the gap between the edges of the molding surfaces is kept constant.
In the molding apparatus of Japanese Patent Laid-Open No. 60-171233, the actuators 11L and 11R for driving the slide cores 10L and 10R are needed, leading to a complicated structure. In the molding apparatus of Japanese Patent Publication No. 63-10100, these actuators are not needed and both the glass regulating portion and portion that allows swelling can be provided. In the latter apparatus, as shown in FIGS. 6 and 7, and 9 and 10, regarding its mold structure, a positional reference 19, or positional references 30 and 31 are formed on the outer surface of the molded optical element. Simultaneously, a continuous annular swell-out portion 20 or 29 is formed to surround the outer surface of the molded optical element. Conventionally, most optical elements molded by glass molding are axially symmetrical. Even if the swell-out portion 20 or 29 annularly surrounds the side surface of the lens at a certain constant height, no particular problem arises.
In recent years, as the types and variations of optical elements increase, a demand for axially asymmetrical lenses increases. Assume that, as shown in FIG. 13, an axially asymmetrical optical element is to be molded (in this case, the lower and upper molds are respectively divided into portions 32a to 32c, and 33a to 33c, and horizontal position regulating members 34 and 35 are provided around the portions 32a to 32c, and 33a to 33c, respectively, to form a space, between position regulating portions 38 and 39, into which an excess 37 of the glass gob flows when press molding is performed in a cylindrical mold 36). As shown in FIG. 14, when a swell-out portion 40 corresponding to the glass gob excess is formed around the molded optical element, this swell-out portion 40 is present very close to a portion 45 (part of the optical functional surface) on, e.g., side surfaces 41 and 42 of the optical element. The deformation amount of glass is large in the swell-out portion 40 and the outermost portion of the swell-out portion 40 is not regulated by the mold during molding. This adversely affects the planar precision of the peripheral portion of the swell-out portion 40.
In this manner, if the edge portion of an optical functional surface 43, or optical functional surfaces 44 and 45 transferred from the molding surface of the upper or lower mold vertically changes and does not have a constant height, the swell-out portion 40 of the glass gob can degrade the planar precision.
In recent years, as the types and variations of optical elements increase, even in molded optical elements such as glass-molded lenses, a demand for molding one having an axially asymmetrical shape increases. When molding an optical element having a general axially asymmetrical shape, it is naturally molded by using an axially asymmetrical mold. During press molding, deformation of the glass material in the horizontal direction is accordingly axially asymmetrical. Assume that such an optical element is to be molded by using a conventionally used mold structure in which the vertical size of the gap, that defines a glass excess, around the edges of the molding surfaces of the upper and lower molds is kept constant. In this case, depending on the relationship between the shape of the glass material before molding and the mold shape, the softened glass may quickly fill even the corners of the mold during molding to result in cracking or cutout during later press molding. Alternatively, the softened glass may not sufficiently fill the corners of the mold even after press molding. The optical functional surface is not perfectly transferred from the molding surface, leading to a molded product having a cut optical functional surface, i.e., a filling defect.